The present invention relates to a diagnosing apparatus for an evaporation purge system and a pressure sensor. More particularly, the invention is directed to a leakage diagnosing apparatus for an evaporation system including a fuel tank, and a pressure sensor suitably used for leakage diagnosis.
An internal combustion engine provided with an evaporation purge system has been in wide use in order to prevent fuel evaporated in a fuel tank from being discharged into the atmosphere. In such an evaporation purge system, fuel evaporated (hereinafter call evaporation) in the fuel tank is temporarily adsorbed by adsorbents filling the inside of a canister. Then, the adsorbed evaporation is discharged through a purge passage to the intake system of the internal combustion engine under a predetermined running condition. However, when the passage in the evaporation purge system is damaged or broken for one reason or another, the evaporation is discharged into the atmosphere. Generally, therefore, leakage diagnosis is carried out for the evaporation system including the fuel tank.
To carry out such a leakage diagnosis, first, the inside of the evaporation system targeted for leakage diagnosis is set in a negative pressure state by using an intake manifold negative pressure or in a positive pressure state by using a pump or the like, and then the evaporation system is hermetically sealed. Then, a change in the pressure of the evaporation system (system inner pressure) is monitored to determine the presence of leakage. However, a problem is inevitable in this case. Specifically, if a relative pressure sensor is used as a pressure sensor for detecting the system inner pressure, fluctuation in an atmospheric pressure causes erroneous determination. The relative pressure sensor is designed to detect a difference between a pressure to be detected and an atmospheric pressure as a reference pressure, i.e., a relative pressure. Thus, when fluctuation occurs in the atmospheric pressure itself, the relative pressure is changed even if the system inner pressure is constant, resulting in the impossibility of discriminating this change from a pressure change caused by leakage. Such fluctuation in the atmospheric pressure occurs because of a change in a car speed (change in a ram pressure), a pressure change during driving on a slope or the like.
In order to solve the foregoing problem, for example, Japanese Patent Application Laid-Open No. Hei. 6-17715 discloses a technology for preventing erroneous determination caused by the effect of a change in the atmospheric pressure by using an atmospheric pressure sensor. This technology is specifically designed to determine the presence of a failure by detecting a system inner pressure from the inside of the evaporation system to the fuel tank, and comparing the changing amount of this detected pressure with a predetermined value. In this case, an atmospheric pressure is detected by the atmospheric pressure sensor and, according to the detected atmospheric pressure, the value of the detected pressure and the predetermined value are corrected.
If the atmospheric pressure sensor like that available in the foregoing conventional art is used, because of detection accuracy required in the leakage diagnosis, the atmospheric pressure sensor must have resolution high enough to detect a very small atmospheric pressure change of 1000 pa or lower. In addition, to deal with various driving conditions including low land and high land driving, the detection range of the atmospheric pressure sensor must be wide. In reality, however, it is not easy to inexpensively manufacture a highly accurate atmospheric pressure sensor capable of satisfying both of such resolution and detection range.
Therefore, an object of the present invention is to provide a highly accurate diagnosing apparatus for an evaporation purge system which is not so easily affected by fluctuation in an atmospheric pressure.
Another object of the invention is to provide a pressure sensor which is not so easily affected by fluctuation in an atmospheric pressure and capable of detecting a pressure in a wide detection range with high accuracy.
In order to achieve the foregoing objects, in accordance with a first aspect of the present invention, there is provided a diagnosing apparatus for an evaporation purge system comprising: hermetically sealing means for setting an evaporation system including a fuel tank in a pressure state different from an atmospheric pressure, and then hermetically sealing the evaporation system during leakage diagnosis; a relative pressure sensor having a reference pressure introduction side, to which a reference pressure is introduced, and a detected pressure introduction side, to which a pressure in the evaporation system is introduced; a valve provided in the reference pressure introduction side; control means for setting the valve in a closed state during the leakage diagnosis; and determination means for determining the presence of leakage in the evaporation system based on a pressure change detected by the relative pressure sensor.
In accordance with a second aspect of the invention, there is provided a pressure sensor comprising: a relative pressure sensor having a reference pressure introduction side, to which a reference pressure is introduced, and a detected pressure introduction side, to which a pressure in a space to be detected is introduced; and a valve provided in a passage for opening the reference pressure introduction side to atmosphere.
In the pressure sensor, when a pressure introduced to the detected pressure introduction side is detected by the relative pressure sensor, a reference pressure introduced to the reference pressure introduction side should preferably be held by setting the valve in a closed state.
According to the present invention, the valve is provided in the reference pressure introduction side of the relative pressure sensor, and the relative pressure of a space to be detected (evaporation system including the fuel tank) is detected while the valve is closed. Accordingly, the detection of the relative pressure can be carried out without any influence of atmospheric pressure fluctuation and with high reliability. Moreover, by carrying out leakage diagnosis for the evaporation purge system based on such detected relative pressure, it is possible to improve the reliability of the leakage diagnosis.